Imaging of cardiac output and regional cerebral blood flow during the mammalian dive response
Ollenberger, Glenn Peter
This thesis utilized radioisotope-labeled tracers to investigate the distribution of cardiac output and regional cerebral blood flow (rCBF) during a mammalian diving response. In addition, the control mechanisms involved in the cerebrovascular response to diving were elucidated. Results indicate that in freely diving animals, the exercising front and hind limbs are not rendered totally ischemic during diving, suggesting that the demands of exercising skeletal muscle partially prevail over the peripheral vasoconstriction associated with diving. Importantly, despite a decreased cardiac output, relative blood flow to the head increased during diving, suggesting that there is maintenance of blood flow to the brain. The pattern of blood flow within the brain was also investigated during voluntarily initiated diving in rats. In twenty-nine of thirty-three brain regions examined, rCBF increased by an average of 1.7-fold, despite a 69.2% decrease in cardiac output. Only some regions of the basal ganglia (caudate putamen-posterior and globus pallidus) and limbic areas (hypothalamus and amygdala) did not increase rCBF significantly during diving. The overall increase in rCBF during diving was determined to be primarily due to a corresponding 20.9% decrease in cerebrovascular resistance. Lastly, the relative contribution of humoral (carbon dioxide) and neural (trigeminal stimulation) inputs on the cerebrovasculature during a simulated dive response were explored. Pre-existing hypocapnia attenuated the increase in rCBF associated with the dive response. This result demonstrated that the decrease in cerebrovascular resistance during diving in small mammals is driven primarily by the progressive hypercapnia associated with asphyxia. Furthermore, the results from this study suggest that trigeminal input does not play any role in differentially modulating the cerebrovascular response to diving. It is concluded that an oxygen conserving response, involving a redistribution of blood flow toward the brain, occurs during voluntarily initiated diving in rats. Although exercising skeletal muscle was shown to modify the peripheral distribution of blood flow during diving, the brain's share of cardiac output increases primarily due to a decrease in cerebrovascular resistance. The resultant increase in CBF during diving was demonstrated to be primarily driven by the progressive hypercapnia associated with asphyxia.